Well, from what I'm getting 15x7's have 5.25" BS while the 15x8's run 5.5" BS. Although the exception are the Moab's which are 5" BS. Not in a hurry as I've got some time before I really need to make a decision. If I have to I could get them mounted to my 15x6's then change over to 15x7's later.
4" BS
Honestly, I wouldn't mind this but 31's are a bit small. Would be nice with my 4.56's though. I'll have to think about that though. I've got 5 of these in a shed out back. Not sure of the condition as they have been out there since '93. Might look funny with some skinny KM2's. Never seen KM2's mounted on them. Anyone else ever come across pics, feel free to post them.